Objective: to investigate the cellular composition of a functionally intact xenopericardial valve in a recipient with acquired mitral defect after long-term implantation. Material and methods. A Uniline bioconduit (BC) («Neocor», Kemerovo) removed from the heart in the mitral position at 7.2 years after implantation was investigated. Heart valve leaflets were fixed in a buffered 4% paraformaldehyde solution and imbedded in paraffin or epoxy resin. Slices made from the paraffin samples were stained with hematoxylin and eosin or underwent immunohistochemical (IHC) examination for typing endothelial cells, smooth muscle cells, macrophages, fibroblasts, and T and B lymphocytes. The epoxy resin-embedded samples were examined using light and scanning electron microscopy according to the original procedure. For this, the samples were ground and polished, then stained with toluidine blue and basic fuchsin or contrasted with uranyl acetate and lead citrate. Results. Different cell types were found in the outer layers of heart valve leaflets. IHC showed that endothelial cells, macrophages, smooth muscle cells, and fibroblasts were present in the samples. A relationship was found between the degree of degenerative changes in the BC surface and the magnitude of cellular infiltration in xenotissue. This paper debates whether impaired integrity of the surface leaflet layers plays a trigger role in structural dysfunctions of the implanted valves and whether BC endothelialization has a protective effect, which can considerably reduce the immunogenicity of xenotussie and prevent the penetration of recipient cells. Conclusion. The paper shows that it is expedient to modify the surface of the heart valve leaflets in order to create favorable conditions for the attachment and function of endothelial progenitor cells.